1. Field of the Invention
This invention relates to an apparatus and method for plugging tubular members and, more particularly, to a tube plug and method for plugging a tube, such as a nuclear steam generator heat exchanger tube.
2. Description of the Prior Art
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective such that a leak either is impending or occurs therein which allows the two fluids to mingle. When this occurs, it is sometimes necessary to plug the tube so that the fluid does not flow through the tube thereby preventing leakage from the tube.
The tube-type heat exchangers utilized in nuclear reactor power plants are commonly referred to as steam generators. When a defect occurs in the tubes of a nuclear steam generator that allows the primary fluid in the tubes to mingle with the secondary fluid outside of the tubes, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radioactive contamination problem. Since the fluid flowing in the tubes of a nuclear steam generator is radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube is plugged so that the primary fluid is not permitted to flow through the tube. This prevents contamination of the fluid surrounding the tubes.
There are several kinds of mechanical plugs that can be used to plug heat exchange tubes. One such device used to plug heat exchange tubes in nuclear steam generators is disclosed in U.S. Pat. No. 4,982,763 issued Jan. 8, 1991 to Klahn entitled "Plug Retainer". The plug retainer for retaining a tube plug comprises a locking cup having a bore therethrough and an externally threaded lower portion adapted to be threadably received in the open threaded end of the tube plug and a cap screw having a threaded shank threadably receivable in the threaded mandrel of the tube plug. The locking cup is crimped onto the cap screw by the use of flutes on the cap screw head. However, the locking cup may inadvertently back out of the tube plug due to vibration during the operation of the steam generator.
Another type of mechanical plug is disclosed in copending application Ser. No. 07/699416 filed concurrently herewith by Keating entitled "Tube Plug and Method for Plugging a Tube" and assigned to the assignee of the present invention. The tube plug for plugging a tube to prevent fluid flow through the tube comprises a shell, an expander member, bolt means to seal the chamber defined by the shell, and locking means for securing the shell, the bolt means, the expander member, and the locking means together. The locking means comprises a tab portion deformable into at least one recess in the bolt means or the shell.
Another type of mechanical plug is disclosed in co-pending application Ser. No. 07/439,118 filed Nov. 20, 1989 by Haberman et al. and assigned to the assignee of the present invention. The tube plug for plugging a tube to prevent flow through the tube comprises a shell, an expander member, and a sealing member to seal the chamber defined by the shell. The sealing member and shell are welded together for locking or fastening the sealing member to the shell to prevent the sealing member and shell from separating. The installation of the sealing member into the shell seals the open end of the shell to prevent coolant from entering the chamber of the shell and exerting pressure on the plug wall, and occupies space within the chamber of the shell to reduce the possibility of residual stresses within the plug wall. Although the tube plug successfully plugs tubes, welding the sealing member and the shell together requires the additional steps of welding and of inspection of the weld during the installation of the tube plug into the tube. Also, if the shell of the tube plug has been recessed a distance into the tube, the sealing member cannot be welded to the shell, and therefore the sealing member and shell cannot be secured together by welding.
Therefore, what is needed is a tube plug which includes means for securing the plurality of members comprising the tube plug together so that the members of the tube plug will not vibrate apart. Also, what is needed is a method for installing the tube plug.